The disclosed invention is directed generally to hybrid multilayer circuit structures, and is directed more particularly to hybrid multilayer circuit structures having ferrimagnetic via structures formed therein.
Hybrid multilayer circuit structures, also known as hybrid microcircuits, implement the interconnection and packaging of discrete circuit devices, and generally include a unitized multilayer circuit structure either formed on a single substrate layer using thick film or thin film techniques, or as a multilayer substrate comprising a plurality of integrally fused insulating layers (e.g., ceramic layers) having conductor traces disposed therebetween. The discrete circuit devices (e.g., integrated circuits) are commonly mounted on the top insulating layer so as not to be covered by another insulating layer or on a insulating layer having die cutouts formed thereon to provide cavities for the discrete devices. Passive components such as capacitors, inductors, and resistors can be formed on the same layer that supports the discrete devices, for example, by thick film processes, or they can be formed between the insulating layers, for example, also by thick film processes. Electrical interconnection of the conductors and components on the different layers is achieved with vias or-holes appropriately located and formed in the insulating layers and filled with conductive material, whereby the conductive material is in contact with predetermined conductive traces between the layers that extend over or under the vias.
Magnetic structures comprised of ferromagnetic and ferrimagnetic materials have been utilized with unitized multiayer circuit structures in the form of discrete external components or structures, which makes the resulting product relatively large and heavy. While ferromagnetic materials have been incorporated in unitized multilayer circuit structures, such materials are conductors which may not be suitable for many applications. Moreover, known processes for incorporating ferromagnetic materials in unitized multilayer circuit structures are incompatible with the formation of unitized multilayer circuit structures using a single sintering process such is used with Low Temperature Cofired Ceramic Tape (LTCC).
Ferrimagnetic materials are typically poor electrical conductors, and that characteristic has been exploited for magnetic shielding or enhancing the performance of inductors and transformer structures. Ferrimagnetic inks and ferromagnetic inks have been developed for use with thick film processes, but the magnetic enhancement realized through the use of ferrimagnetic inks and ferromagnetic inks is limited by the achievable thickness of the ink printing since performance in a closed magnetic path is dependent on the cross sectional area of the magnetic material. Moreover, such known ferrimagnetic inks and ferromagnetic inks are incompatible with LTCC or similar processing due to shrinkage profile dissimilarities between the known inks and LTCC, which causes warping and buckling of the multilayer circuit structure during firing.